BOLIVAR
[Geology]
The geologic setting for the project is a volcano-sedimentary assemblage of Cretaceous age that comprises flows and tuffs of andesite and basalt intercalated with greywacke, limestone and shale beds. Two distinct periods of volcanic activity have been recognized.
The first is referred to as the Lower Volcanic Series (LVS) and is of Upper Cretaceous to Lower Tertiary age (80-90 to 40-50 Ma). It comprises a wide variety of intrusive and extrusive rocks, but is mainly composed of andesitic agglomerates, flows and tuffs grading upward into a felsic volcanic assemblage. The overall thickness of the LVS is at least 750 m in the Bolivar area, but the contact between the Cretaceous "basement" and the LVS is not exposed.
Basement and LVS rocks are intruded by plutonic bodies of intermediate to felsic composition of Upper Cretaceous to Lower Tertiary age. Granodiorite bodies are of two epochs, one at 85.0 Ma and a second at 45.3 Ma. Quartz diorites have ages ranging between 47.9 and 28.3 Ma. A 40-km-wide linear belt of these intrusive bodies is exposed in windows throughout the State of Chihuahua. The rocks are relatively fresh but zones of alteration, hornfels, marble and skarn are developed adjacent to some intrusions.
The LVS is overlain by a widespread capping of continental rhyolitic and dacitic ignimbrites and tuffs named the Upper Volcanic Series (UVS). The UVS was deposited between 30 and 26 Ma, and no mineralization is known in the UVS in this region. All known mineralization in this region occurred during the time interval between the LVS and UVS.
The rocks of the UVS tilt to the east but the area is relatively unfolded, and deformation in the project areas is controlled by shear faults, listric faults, and tensional fractures. All units are strongly block-listric-faulted but stratigraphic displacements do not exceed 200 meters. Large extensional blocks occur in the faulted areas.